port ARGBToRGB565 and ARGB1555 to AVX2.  Enable functions that use ARGBToRGB565 AVX2 code.  Add ARGBToRGB565Dither function.
BUG=403
TESTED=local windows build
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/42109004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1302 16f28f9a-4ce2-e073-06de-1de4eb20be90